The terms "open content" and "open educational resources" describe any copyrightable work (traditionally excluding software, which is described by other terms like "open source") that is either (1) in the public domain or (2) licensed in a manner that provides everyone with free and perpetual permission to engage in the 5R activities:
There is no shortage of places to find OER. These are a few places to start your search, but feel free to use other resources to your advantage. Bigger libraries often have a librarian dedicated specifically to open education resource pursuits, so checking out their guides could be fruitful.
Just as you wouldn't use a textbook in a course without evaluating it, you shouldn't use open educational resources without a thorough evaluation. In Abbey Elder's book (linked above) she provides an entire chapter on Evaluating OER. She also links to a helpful Google Doc checklist adapted from the criteria on the Affordable Learning Georgia.
Here are a few other resources for evaluating OER.
Vogel Library, Wartburg College | 100 Wartburg Blvd, Waverly, IA, 50677 | Phone: 319-352-8500 | Email: firstname.lastname@example.org